Does proliferate affect loyalty counters on planeswalkers?

Proliferate (CR 701.27) lets you choose any number of permanents and/or players that already have counters on them, then put one additional counter of each kind already there on each chosen permanent or player. Loyalty counters are simply a counter type, so they are fully eligible targets for proliferate.

Planeswalkers enter the battlefield with a number of loyalty counters equal to their printed loyalty (CR 306.5b). Once they are on the battlefield with those counters, proliferate can add one more loyalty counter to each planeswalker you choose. This effectively gives the planeswalker one extra loyalty, which can be very powerful mid-game.

There is one key condition: the planeswalker must already have at least one loyalty counter on it. If a planeswalker somehow has zero loyalty counters (which would normally cause it to be put into the graveyard immediately under CR 306.9), it cannot be chosen for proliferate because it has no counters of that type present.

Concrete example: You control a Jace, the Mind Sculptor with 3 loyalty counters. You cast Karn's Bastion and activate its proliferate ability. You choose Jace. He now has 4 loyalty counters. This can let him survive an attack or reach an ultimate faster than your opponent expected.

Relevant rules: CR 701.27 (Proliferate), CR 306.5b (planeswalkers entering with loyalty counters), CR 306.9 (planeswalker with zero loyalty leaves the battlefield).